Steam turbine control systems often incorporate electro-mechanical pilot-valve actuator assemblies to regulate hydraulically-driven actuator pistons that modulate steam valves, thereby controlling turbine speed. However, it is becoming more popular to use direct electro-mechanical actuation of the steam valves themselves, since they are fully capable of satisfying force and speed requirements.
In case of electro-mechanical pilot-valve actuation, some of the prior art systems can be applied to provide trip response upon complete power interruption. In case of the direct actuation of the steam valve, however, using electro-mechanical actuators, a complete interruption of electrical power to the electromotor makes this solution unusable in some industries, especially in power utility industries where the requirement is that there be a trip response of the steam valves for many circumstances. The application of the prior art systems, though is technically impractical.
For example, it is well known that the drawback of electromagnetic actuation is that on brief interruptions of electrical power, the actuator causes a trip response to the pilot valve, whereas a drawback of electromotor actuation is that on complete interruptions of electrical power the actuator cannot independently cause a trip response of the pilot valve on demand.
Thus, there is a need for a method and apparatus to provide a trip response of electrically actuated steam turbine control valves in the case of primary electrical power source interruption or failure of any primary control system electrical component.